1. Field of the Invention
The present invention generally relates to a torque sensor adapted to detect torque acting on a shaft. More particularly, the invention is related to a torque sensor suitable for application on an electric motor-driven power steering arrangement of an automobile.
2. Description of Related Art
Electro motor-driven power steering systems have been developed in recent years to help assist in the steering of an automobile. The power steering arrangement referred to above requires an electric motor mounted to a steering system which is rotated in accordance with the detected torque applied to the steering wheel.
Torque sensors detecting the torque applied to the steering wheel as shown in FIGS. 1 and 2, have been proposed by the inventors of the present invention and others (Japanese Utility Model Application No. 63-54841) (54841/1988). FIG. 1 is a half cross sectional view of the prior art torque sensor wherein an upper shaft 11A connected to the steering wheel is coaxially coupled to a lower shaft 11C connected to the steering system, via a torsion bar 11B. The upper shaft 11A is rotatably supported by a cylindrical casing 25 secured to the automobile body (not shown) via a bearing 26. A first sleeve 15b made of a non-magnetic material is securely fitted on the outer periphery of the upper end (right side in the drawing) of the lower shaft 11C, with a first cylinder 18 made of a magnetic material being securely fixed to the outer periphery thereof.
One end face at the side of the lower shaft 11C of the first cylinder 18 is a flat surface perpendicular to the center of the lower shaft 11C. The other end face at the side of the upper shaft 11A of the first cylinder 18 has many rectangular toothed parts 18a, 18a, . . . is perpendicular to the center of the lower shaft 11C.
Similarly, a second sleeve 15a made of a non-magnetic material is securely fitted on the outer periphery of the lower end (left side in the drawing) of the upper shaft 11A, and second and third cylinders 17 and 14 made of magnetic material is fixed on the outer periphery thereof. Further, a gap forming member 10, which is ring-shaped and of narrow width made of non-magnetic material such as brass, aluminum, austenite stainless steel or the like is intervened between the second and third cylinders 17 and 14, and exteriorly fitted on the outer periphery of the second sleeve 15a.
The third cylinder 14 has the same configuration as that of the first cylinder 18, and is mounted to the reverse direction of the first cylinder 18.
The opposite end face of the second cylinder 17 has toothed parts 17a, 17a . . . and 17b, 17b . . . formed in the same configuration and with the same pitch as the aforementioned toothed parts 18a, 18a, . . . . Each of the toothed parts 17a, 17b is met on the same line parallel to the center of the cylinder 17. Moreover, the cylinders 18 and 17 are so positioned in the rotational direction thereof that the toothed parts 18a and 17b confront each other at the position half the tooth width when no torque acts on the torsion bar 11B. The toothed parts 17b, 17b . . . of the second cylinder 17 are spaced at an appropriate distance from the toothed parts 18a, 18a . . . of the first cylinder 18.
The cylinders 17 and 14 are positioned so that the toothed parts 17a and 14a confront each other at the position half the tooth width thereof, in the similar manner as the toothed parts 17b and 18a, when no torque acts on the torsion bar 11B.
There is a stopper 50 in the form of the narrow strip projecting parallel to the center of the upper shaft 11A on the outer peripheral surface at the lower end of the shaft 11A. At the upper end of the lower shaft 11C, a stopper guide groove 51 is notched with an appropriate length in the peripheral direction of the inner peripheral surface of an opening formed concentrically with the center of the lower shaft 11C. The above stopper 50 is engaged into the stopper guide groove 51. The arc length of the stopper guide groove 51 are set so that the upper and lower shafts 11A and 11C are able to rotate relatively, for example, by approximately half the tooth width of the toothed part 17b.
Inside the casing 25 are securely fitted cylindrical members 19A and 19B made of magnetic material which respectively form peripheral grooves. The cylindrical member 19A faces the confronting part of the second and third cylinders 17 and 14, and the cylindrical member 19B faces the confronting part of the first and second cylinders 18 and 17. A first electromagnetic coupling detection coil 32 and a second electromagnetic coupling detection coil 31 are wound around in the peripheral grooves of the cylindrical members 19B and 19A, respectively, so that the cylinders 18 and 17 are electromagnetically coupled to each other by the first electromagnetic coupling detection coil 32, while the cylinders 17 and 14 are electromagnetically coupled to each other by the second electromagnetic coupling detection coil 31.
In the prior art torque sensor mentioned above, since the first, second and third cylinders 18, 17 and 14 as well as the cylindrical members 19A and 19B are fixed respectively as described hereinabove, the positioning accuracy of these cylinders in the axial direction depends on the accuracy of the size of each component thereof. Therefore, if the sizing accuracy is poor, the cylinders 18, 17 and 14 burdened with an increase in the pressing-in load in the radial direction thereof, resulting in an increase of the stress. Moreover, in such a case as above, the cylindrical members 19A and 19B rattle in the axial direction, bringing about a problem such that an abnormal output may occur because of the increase in the stress due to the temperature change or the vibrations caused by the rattling.
Moreover, although the toothed parts 17a, 17b are provided at the opposite end faces of the second cylinder 17, it is considerably difficult to position the toothed parts at one end face relative to those at the other end face, without generating positional deviations therebetween. This problem must be solved by manufacturing techniques.
The first, second and third cylinders 18, 17 and 14 are fixed by being pressed into the lower and upper shafts 11C and 11A via the non-magnetic sleeves 15b and 15a. According to this method, however, positioning the cylinders 18, 17 and 14 in the axial direction cannot be achieved.
Further, in the prior art torque sensor having the above-described construction, the third cylinder 14 can be positioned in the axial direction by the gap forming member 20 interposed between the second and third cylinders 17 and 14. However, so as to bring the third and second cylinders 14 and 17 into the electromagnetically-coupled state equal to the state of the second and first cylinders 17 and 18, much troublesome adjusting work is required that the third cylinder 14 is secured to the second sleeve 15a simultaneously when the cylinder 14 is pressed against the cylinder 17 via the gap forming member 20 while the cylinder 14 is being rotated for adjustment.